As line widths of semiconductor devices decreases, desire for low resistance polycides with low aspect ratios has led to the consideration of various polycide materials other than the conventional tungsten or titanium silicide. For example, such polycide materials may include cobalt silicide, tungsten nitride, and other refractory metal silicides. To use such materials, effective methods for forming polycide structures must be developed.
Cobalt silicide, in particular, due to its low bulk resistivity and small grain size, allows for low resistivity to be preserved for fine line widths of polycide structures. However, cobalt silicide is a difficult material to volatilize and thus dry etch or remove when patterned. As such, the use of different refractory metal silicides for forming polycide structures, in particular, cobalt silicide, has been focused on its use in the salicide process where both transistor gate and source/drain diffusion regions in the active area of devices, such as memory devices, are simultaneously silicided, such as shown in the prior art figures of FIG. 1A and FIG. 1B.
As shown in FIG. 1A and FIG. 1B, field oxide regions 17, gate region 12 including spacers 16, gate oxide 14, and polysilicon 18, and also source and drain regions 22, 24, are formed on substrate 10. A layer of refractory metal, for example cobalt, is deposited such as by sputtering on these previously fabricated regions. The structure being fabricated is then heated to a temperature at which the refractory metal forms metal silicide regions 26, 28, and 30 in the source region 22, drain region 24, and with polysilicon 18 of gate region 12, respectively. Thus polycide line 13, i.e., a word line including a polysilicon portion 18 and a metal silicided portion 30, is formed. However, metal silicide is not formed over the oxide regions, such as spacers 16 and field oxide regions 17. The unreacted refractory metal 20 over such oxide regions, such as spacers 16 and field oxide regions 17, is then removed. An insulating layer 32 is formed over the structure with a contact hole being formed for connection of a contact 34 to the metal silicide region 30 of polycide word line 13.
However, in many circumstances, such as in the fabrication of dynamic random access memories, it is undesirable to silicide the source region 22 and drain region 24. Such undesirability is due to decreased line widths as it is difficult to accommodate such salicided source and drain regions in processes which require minimal gate junction/channel leakage. Such leakage may occur due to source and/or drain diffusion into the junction/channel region of the gate.
A conventional process flow for patterning a polycide line without saliciding the drain or source regions is shown in the prior art FIGS. 2A-2D. FIG. 2A shows a structure including field oxide regions 42 and gate oxide region 44 isolated in the active area therebetween. As shown in FIG. 2B, a polysilicon layer 46 is formed over the field oxide regions 42 and gate oxide 44 followed by a layer of refractory metal 48, such as cobalt. After the structure, including the polysilicon layer 46 and refractory metal layer 48, is annealed to form metal silicide layer 47 (FIG. 2C), such as cobalt silicide, the polycide line 51 (FIG. 2D), i.e., word line, is patterned using photoresist 50. The cobalt silicide or metal silicide layer 47 and polysilicon layer 46 are then etched to form the polycide word line 51 including metal silicide region 52 and polysilicon region 53. Thereafter, spacers may be formed at the walls of the polycide word line. Gate oxide 44 may be etched to form gate oxide region 45 and various implants performed to provide drain and source regions 54 and 56 of the structure as shown in FIG. 2D. However, the refractory metal reacts with all of the polysilicon layer 46 and not just at the region where the polycide structure is to be formed. As it is difficult to etch some refractory metal suicides, such as, for example, cobalt silicide, due to difficulty of volatilizing and thus dry etching the silicide, this conventional process is problematic.
Accordingly, there is a need in the art for forming polycide structures, such as lines, and in particular, polycide structures utilizing cobalt silicide, which overcomes the difficulty in etching the refractory metal silicide in the formation process. Such a polycide structure formation process should be usable in various processes such as in the formation of a polycide word line or bit line with no need to salicide the drain and source regions of devices being fabricated. The present invention overcomes the problems as described above and overcomes other problems as will become apparent to one skilled in the art from the description below.